1. Field of the Invention
The present invention is related to the field of oil well perforating systems. More specifically, the present invention is related to systems for transferring detonating signals from an explosive initiator to shaped charges in a well perforating gun assembly.
2. Description of the Related Art
Wellbores drilled through earth formations for extracting oil and gas are typically completed by coaxially inserting a steel pipe, called casing, into the wellbore. The earth formations are put in hydraulic communication with the wellbore by making holes, referred to as perforations, in the casing. Perforations are typically made in the casing by detonating explosive shaped charges inside the casing at a depth adjacent to the earth formation which is to produce the oil and gas: Shaped charges are configured to direct the energy of an explosive detonation in a narrow pattern, called a "jet", which creates the hole in the casing.
The shaped charges are initiated by a detonating signal which is transferred from an initiator, through a hollow metal, cloth or plastic tube filled with high explosive. The initiator can be a lead-azide type electrical blasting cap, an electrically-activated exploding bridegewire ("EBW") initiator, an electrically activated exploding-foil initiator ("EFI") or a percussively-activated explosive initiator. The explosive-filled tube is generally referred to as "detonating cord". A type of detonating cord known in the art is sold by the Ensign-Bickford Company under the trade name "PRIMACORD".
The percussively-activated explosive type initiator is typically used in oil well perforating systems known as "tubing conveyed" systems. As is known to those skilled in the art, tubing conveyed perforating systems are used to create perforations in oil wells without requiring insertion of an electric wireline into the wellbore. As is also known by those skilled in the art, creating perforations without a wireline inserted into the wellbore enables initiation of the shaped charges, and consequently creation of the perforations, while the wellbore has an internal pressure significantly less than the fluid pressure of the oil and gas within the earth formation, so that the perforations can have increased hydraulic efficiency.
The percussively-activated initiator in a tubing-conveyed system can be activated by dropping a rod or "bar" from the earth's surface, through the wellbore, onto the initiator. Another version of percussive initiator, called a "pressure activated" initiator, includes a piston restrained by shear pins inside a housing. The housing is sealed against wellbore pressure on one side, and the back side of the piston is exposed to the pressure present in the wellbore through the open end of the housing. Fluid pressure can be applied to the wellbore at the earth's surface to the wellbore. The pressure is communicated to the back side of the piston until the hydraulic force on the piston exceeds the shear strength of the pins. When the shear pins break, the piston is released so that it can travel and strike the initiator, initiating the explosion in a manner similar to the dropped bar initiator.
The initiators known in the an occasionally fail to detonate the shaped charges because the high explosive in the initiator and/or the detonating cord burns instead of exploding. This type of failure is referred to as a "low order" failure. A particular difficulty with tubing-conveyed systems which undergo low order failure is that a booster explosive, which transfers the detonating signal from the detonating cord to the top of a gun carrier containing the shaped charges, can be damaged by the low order burning of the detonating cord. If the booster explosive is damaged by low order failure, then the entire gun carrier must typically be retrieved from the wellbore, disassembled and reloaded, which can be difficult and expensive.
Tubing-conveyed perforating systems known in the art typically provide a second initiator so that if the first initiator and its associated detonating cord fail to detonate the shaped charges, the failure can be overcome by activating the second initiator. Such systems are referred to as redundant firing head systems. A drawback to the redundant firing head systems known in the art is that low order failure of the first initiator can damage the booster explosive, so that even if the second initiator detonates properly, the detonating signal may not transfer to the shaped charges.
It is known in the art to prevent damage to the booster explosive by providing a barrier between the booster explosive and the detonating cord. The barrier can be penetrated by a shaped charge disposed at the end of the detonating cord which can explosively penetrate the barrier only upon proper "high-order" initiation of the detonating cord. Such a barrier system is described, for example in U.S. Pat. No. 4,650,009 issued to McClure et al. The system in the McClure et al '009 patent, however, is intended to be used either with a single initiator and detonating cord, or to transfer the detonating signal along a single explosive path through serially connected gun sections. The system in the McClure et al '009 patent is not suitable for use in redundant firing head systems because it only includes a single shaped charge. Low-order failure of the first initiator could damage the shaped charge so that even a proper high-order detonation of the second initiator would fail to cause detonation of the shaped charge, preventing normal detonation of the gun assembly.
Accordingly, it is an object of the present invention to provide a redundant firing head perforator system that can detonate shaped charges even after a low-order failure of the first explosive initiator and/or detonating cord.